How do atoms exist 1

Molecules in space

Research Report 2014 - Max Planck Institute for Nuclear Physics

Grussie, Florian; O'Connor, Aodh P .; Kreckel, Holger

Astrolabe research group (ERC Starting Grant, Holger Kreckel)
Stored and Cooled Ions Department (Klaus Blaum)

Between the stars is a highly diluted mixture of atoms and molecules with a small fraction of macroscopic dust particles. In the denser areas, the interstellar clouds, where stars and planets are born, modern telescopes have revealed surprising molecular diversity and even found complex organic molecules. At the Max Planck Institute for Nuclear Physics, laboratory experiments that are unique worldwide are being developed to try to simulate and understand the formation of molecules under conditions that are as extreme as those in interstellar space.

The chemistry of space

In interstellar clouds there are conditions that appear very extreme compared to terrestrial conditions. On the one hand, the particle densities are so low that they can hardly be achieved even in the most modern ultra-high vacuum equipment. On the other hand, the temperatures drop very close to absolute zero, down to –263 ° C (or 10 K). Combined, these boundary conditions ensure that interstellar chemistry takes completely different paths than we are used to here on Earth. Due to the low temperature, most of the processes that require activation energy are excluded from the outset. This means that a huge class of reactions is not even available.

Despite these adverse circumstances, more than 180 different molecules have already been discovered in space. In recent years in particular, major projects such as the Herschel Space Telescope and the Atacama Large Millimeter / submillimeter Array (ALMA) have found increasingly complex molecules in the most inhospitable environments. It is also becoming increasingly clear that a thorough understanding of the formation of molecules in the interstellar medium and in protoplanetary disks - the birthplaces of planets - will be necessary in order to understand the origin of life on our own planet. The rapid progress in the field of extrasolar planets underscores this point, as in the near future the search for molecules as signs of life in planetary atmospheres outside our solar system will come into focus.

As a consequence of these observations, the question arises of how interstellar molecules can arise and survive under such hostile conditions. Processes between charged molecules, so-called molecular ions, and neutral atoms and molecules were identified years ago as the key to molecular diversity [1]. In these ion-neutral reactions, an attractive force arises from the fact that the charged collision partner polarizes the neutral one. Therefore, a reaction becomes much more likely. In addition, it has been found that many ion-neutral reactions do not have an activation barrier and can therefore take place effectively even at the lowest temperatures. illustration 1 shows some typical reaction chains that illustrate how more and more complex molecules are created through ion-neutral reactions, up to and including the formation of water and organic compounds. Modern astrochemical models, with which one tries to understand the conditions in interstellar clouds, meanwhile contain several thousand reactions in which very frequently charged molecules play a role. Only a fraction of the relevant reaction rates have so far been measured in the laboratory - mostly exclusively at room temperature, which does not allow any conclusions to be drawn about any temperature dependencies. Some of the most fundamental and frequent chemical reactions in the universe are still new experimental territory and theoretically poorly understood. To remedy this, the Max Planck Institute for Nuclear Physics (MPIK) is developing new experimental methods to measure astrochemical processes under interstellar conditions in the laboratory. The core of laboratory astrophysics at the MPIK is the cryogenic storage ring (CSR) [2].

The cryogenic storage ring - an ice-cold laboratory

Worldwide, there are several electrostatic storage rings in operation or under construction, which have the advantage over storage rings with magnetic deflection that they can simply store charged molecules with much larger masses. Therefore, these systems are also suitable for experiments with heavy and complex molecules up to biologically relevant species. The CSR (Fig. 2) the most ambitious project of its kind worldwide. The entire inner vacuum structure, with a circumference of 35.4 m, is cooled to less than 10 K (–263 ° C) by a closed helium circuit. This enables experiments with cold stored molecular beams that are effectively shielded from the thermal radiation of the laboratory. The low temperatures are also important in other ways. In the CSR, all remaining residues of atmospheric gases will freeze on at particularly cold points. This creates an extreme vacuum in the inner chambers of a quality that has never been created in such a large system. After initial tests, it is hoped that densities will only be a few thousand particles per cubic centimeter. This corresponds to a pressure of <10-16 Atmospheres. This ultra-pure, cryogenic environment actually comes close to interstellar conditions and is therefore ideally suited to simulating the formation of molecules in space in the laboratory.

Neutral atomic beams for targeted experiments

Reactions between molecular ions and neutral atoms pose a particular challenge because both reaction partners must first be prepared in a complex manner and are very reactive. Only the noble gases and mercury are monatomic under terrestrial conditions. So it goes without saying for us that oxygen tanks O2 instead of atomic O Likewise, hydrogen and nitrogen are present as diatomic H2 and N2 in front. Solid forms such as graphite or diamond are found in pure carbon. In space, however, atomic hydrogen (H), oxygen (O) and carbon (C) are among the fundamental components of interstellar clouds, and reactions with these neutral atoms in the ground state are of enormous importance. In order to simulate these processes under realistic conditions, the CSR is coupled with a special atomic beam experiment. For this purpose, negative ion beams of the desired element are first generated in an ion source. From a cesium sputter source, for example, intense H-, D-, C- and O-Extract rays. At an energy of up to 300 keV, the negative ions are first mass-selected in a magnet and then guided in the direction of CSR by electrostatic deflectors. However, before the negative ions reach the storage ring, they pass through a strong laser field (Fig. 3) [3]. Here the external electron is removed by the photons in up to 10% of the ions and thus a neutral atom in the electronic ground state is generated [4]. The remaining negative ions can simply be electrostatically deflected and collected in a Faraday cup, while the neutral atomic beam flies on unaffected into the CSR.

As collision partners for the stored molecular ions, the neutral atoms are superimposed with the circulating beam of the CSR. The speeds of the atoms and ions are precisely matched to one another and this leads to ion-neutral collisions in an ultra-high vacuum and at temperatures around 10 K, i.e. under real interstellar conditions. An important example process is the incorporation of heavy hydrogen (deuterium) into triatomic hydrogen molecules, according to H.3+ + D → H2D.+ + H. That deuterated H2D.+-Product is heavier than the original H.3+Ion and is therefore separated from the stored beam immediately after its formation by the electrical fields of the CSR. With particle detectors at suitable points in the CSR, the H2D.+Ions are then individually detected and counted to determine the rate of reaction. The above reaction is especially important in diffuse interstellar clouds, as it can introduce deuterium atoms into the chemical network via the highly reactive triatomic hydrogen ion. In subsequent reactions, the deuterium atoms can then be passed on to other reaction partners, which can ultimately lead to an accumulation of deuterium in interstellar water. The D / H ratio in space plays an important role in the question of the origin of water in our own oceans on earth. The fundamental deuterium exchange process described above has never been measured under interstellar conditions, and estimates have to be used for model calculations.

A similar problematic situation arises with the formation of simple organic molecules. Since the production of atomic carbon beams has so far been associated with great difficulties, there are practically no reliable experimental reaction rates for the entire class of reactions between neutral carbon atoms and charged molecular ions, although these processes are considered to be important pathways for the beginning of organic chemistry in model calculations Enter space.

Next generation laboratory astrophysics

To study all these questions through precise, energy-resolved measurements - in addition to atoms also with electrons as collision partners - the CSR will be available in the near future as a globally unique laboratory. Together with the great advances in the field of molecule observation, and the potential of the ALMA telescopes in particular, an exciting time is ahead in the field of molecular astrophysics.

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